TOKYO (Reuters) - Japan's SoftBank Corp said on Tuesday its third-quarter operating profit slipped 5.9 percent, missing estimates, as the ambitious mobile telecom firm continues to soak up costs in its drive to turn around loss-making U.S. unit Sprint Corp.

SoftBank, which bought control of Sprint for more than $20 billion in 2012, said October-December operating profit was 191.39 billion yen ($1.6 billion), down from 203.46 billion yen a year earlier, restated from 209.16 billion yen in line with a change in accounting standards. The was well below the 219.08 billion yen average of estimates made by five analysts, according to Thomson Reuters StarMine.

The company, Japan's third-biggest mobile carrier by subscriber numbers, kept its full-year profit forecast for the fiscal year ending March unchanged.

Earlier this month, Sprint reported revenue for the quarter ended December fell less than expected, as the U.S. mobile provider attracted more subscribers by cutting prices and offering promotions.